Childhood obesity is increasing nationwide, is associated with serious health complications, and tracks into adulthood. However, controlled obesity prevention studies remain scarce. Behavioral-economics theory offers a novel framework for conceptualizing and designing such interventions. To this end, we propose a randomized controlled trial that tests the effects of a "Family-Based Food Substitutions" intervention - with and without a Home-Food-Provisions component - on long-term weight change in children at risk for obesity. Based on behavioral-economics theory, we predict that increasing fruit and vegetable intake will substitute for the intake of untargeted energy dense foods and thereby reduce weight gain in children at-risk for obesity. This effect is predicted to be enhanced among families provided home provisions of targeted fruits and vegetables. Participants will be 180 families of diverse ethnic background, with the target child being 4 to 6 years old and "at-risk" for obesity. Families will be randomized to one of the following three treatment groups: (1) Minimal Intervention Control; (2) Family-Based Food Substitutions (FBFS); or (3) FBFS with Home-Food Provisions. There will be 60 families per condition. Families assigned to the control group will receive nutrition education. Those assigned to the FBFS group will be trained in behavioral strategies that target increased child fruit and vegetable intake (e.g., role modeling and contingency management training for parents). Families assigned to the FBFS with Home-Food-Provisions group will receive the aforementioned package plus supplies of targeted fruits and vegetables to enhance their home accessibility. The primary outcome measure will be short-term and 2-year changes in the children's body composition. Secondary outcomes will include measures of dietary intake, parental food preparation skills, and fruit and vegetable accessibility. Mediator analyses will elucidate the mechanisms by which the intervention exerts its effect on changes in body composition. This investigation is designed to provide new insights into environmental manipulations that will induce food substitutions compatible with obesity prevention, while advancing behavioral-economics theory.